1. Field of the Invention
The present invention relates to an apparatus and method for hot drawing electroresistance welded steel pipe, welded steel pipe, seamless steel pipe, etc., more particularly relates to an apparatus and method for production of tapered steel pipe comprised of steel pipe with an outside diameter gradually changed in the axial direction.
2. Description of the Related Art
Tapered steel pipe comprised of steel pipe with an outside diameter gradually changed in the axial direction and with ridge of the steel pipes inclined with respect to its axis is being used for road light poles etc. Apparatuses for producing such tapered steel pipe by drawing (or spinning) are, for example, disclosed in Japanese Patent Publication (A) No. 10-24323, Japanese Patent Publication (A) No. 11-197755, Japanese Patent Publication (A) No. 2002-192225, Japanese Patent Publication (A) No. 2002-292432, and Japanese Patent Publication (A) No. 2002-292433.
These apparatuses for production of steel pipe hold the two ends of the steel pipe on shafts on carrier and move it in the axial direction while rotating it to draw it into a taper by an intermediate working roll. Note that the steel pipe is heated as a whole in a heating oven or is partially heated by a heating apparatus set at the entry side of the working roll. The drawing is performed hot at a temperature of several hundred degrees centigrade.
Normally, in an apparatus for production of a tapered steel pipe, as shown in FIG. 15, the working roll 28 is set so that its shaft is parallel to the axis of rotation of the steel pipe P. In this case, if hot drawing (or hot spinning) the pipe at a high speed, there were the problem that the outside shape did not become round but ended up becoming polygonal and the problem of a variation in thickness and due to this a large amount of orange peel surface ended up occurring.
Further, the angle (taper angle) of the ridges of the tapered steel pipe with respect to the axial direction is determined by the speed of advance/retraction of the working roll in the direction from the outer circumference of the steel pipe to the center axis (radial direction) and the speed of movement in the longitudinal direction (axial direction) of the steel pipe. Normally, the steel pipe is pulled by one carrier, while the other carrier moves trailing it. The other carrier imparts a suitable tension to the steel pipe for improving the shape of the tapered steel pipe. Further, the steel pipe is imparted with a rotational force by a shaft placed on either of the carriers, while the shaft of the other carrier follows the rotation of the pipe. In this way, the steel pipe rotates at a constant speed while contacting the working roll and being drawn. Note that the working roll is not imparted any drive force and is a freely rotating type which rotates by contact with the steel pipe.
If using such a production apparatus to, for example, draw a long electroresistance welded steel pipe with a total length of over 10 meters at a high speed, as shown in FIG. 16, the seam line of the steel pipe sometimes becomes twisted. If this twisting of the seam line becomes severe, the pipe easily becomes polygonal in cross-section, twisted, bent, or otherwise defective. The reason for the twisting of the seam line is the difference in the peripheral speed of the worked part of the steel pipe and the peripheral speed of the working roll arising due to the fact that the peripheral speed of the large inertia moment working roll cannot keep up with the change in the peripheral speed of the worked part of the steel pipe. Therefore, when drawing a long steel pipe, the speed of movement of the steel pipe in the axial direction has to be made slower and the change in the peripheral speed of the worked part of the steel pipe has to be made smaller and therefore there was the problem of a drop in the productivity.
In particular, the working defect rate is high in the case of a stock pipe (a raw workpiece) with an outside diameter of 160 mm and in the case of a tapered steel pipe with a maximum value of the outside diameter of 150 mm or less. This is because if the diameter of the steel pipe becomes smaller than the diameter of a working roll, the resistance to cross-sectional deformation of the steel pipe becomes relatively weak and the working instability increases. Further, the working defect rate is also high in the case of drawing a stock pipe with a thickness of less than 4.0 mm and the case of producing a tapered steel pipe with a maximum value of thickness of less than 4.0 mm. This is believed to be because if the thickness becomes smaller, the resistance to cross-sectional deformation of the steel pipe becomes weak in absolute terms and again the working instability increases. Further, even when the minimum value of the outside diameter of the tapered steel pipe is 20% or less of the outside diameter of the stock pipe, the working defect rate is high. This is believed to be due to the high taper rate, that is, the increase in drawing, and therefore the increase in external force and the increase in working instability itself.
Further, the optimum working temperature when drawing the steel pipe (optimum working temperature) differs depending on the type of steel. Control to a suitable range is preferable. If calculated from the rate of change of strength of the steel pipe, the heating apparatus is particularly preferably controlled so that the temperature of the steel pipe after heating to when reaching the working roll becomes a range of the optimum working temperature ±20° C. Conventionally, when drawing steel pipe hot, the output of the heating apparatus has been adjusted so that the temperature of the steel pipe becomes the optimum working temperature at the exit side of the heating apparatus.
In general, anti-swing rings or other devices are provided between the heating apparatus and the working roll of a tapered steel pipe production apparatus, so the distance between the heating apparatus and working roll is made approximately 600 mm. Further, the speed of movement of the steel pipe in the axial direction when producing a tapered steel pipe is 0.5 to 0.7 m/min. Therefore, after the steel pipe is heated at the heating apparatus, it takes over 1 minute or so until reaching the working roll. Air cooling reduces the temperature of the steel pipe to as low as 100° C. When the temperature drops sharply in this way, the response time in temperature control of the heating apparatus is long, so control of the temperature is difficult. Therefore, the method of adjusting the output of the heating apparatus so that the temperature of the steel pipe at the exit side of the heating apparatus becomes the optimum working temperature is suitable for production of tapered steel pipe.
As opposed to this, it is also possible to assume that the amount of temperature drop due to the air cooling in the interval from the end of heating the steel pipe to the drawing operation is constant, set a constant temperature drop constant, and control the temperature of the steel pipe at the exit side of the heating apparatus. However, to secure working stability, generally the working speed is set constant. For example, when the amount of drawing becomes larger, inversely proportional to this, the speed of the steel pipe at the entry side, that is, the speed of the steel pipe passing through the heating apparatus, becomes slower. Therefore, along with the drawing work, the temperature drop becomes greater, so even if setting the temperature drop constant, it is difficult to maintain a suitable working temperature.
In addition, even a change in the temperature around the production apparatus due to the season or time has an effect on the temperature drop constant. Further, the steel pipe is cooled by contact with the water-cooled working roll, so even a change in volume of the steel pipe resulting from the drawing conditions has an effect on the temperature drop constant. From the above, with the method of setting a constant temperature drop constant and controlling the temperature at the exit side of the heating apparatus, maintaining a suitable working temperature is difficult. Still further, when producing a steel pipe with a non-constant taper rate, hunting inevitably ends up occurring if performing control with the target temperature set constant.
From the above, adjusting the output of the heating apparatus so that the temperature of the steel pipe when reaching the working roll becomes the optimum working temperature (target temperature) is extremely difficult. In the past, control of the heating temperature has been dependent on the experience of the workers. Therefore, stable operation has not been able to be secured all the time. The steel pipe is liable to be overheated at the exit side of the heating apparatus, the deformation resistance of the steel pipe is liable to fall, deformation is liable to occur before reaching the working roll, and other trouble is liable to occur.